The present invention relates to pumps in general, and more particularly to high-pressure piston pumps including one or more pistons and having adjustable output rates.
It is already known to use plunger-type high-pressure pumps, in which the pressure-volume ratio remains constant, in various applications, such as in hydrodynamic cleaning operations, in hydraulic or jet mining of coal or similar substances, in operation of hydraulic presses, as process pumps in diverse treatment installations, as homogenizers, and the like. In these pumps, the output rate (the amount being pumped per unit time) remains the same as long as the speed at which the pump is driven is maintained constant. Consequently, when it is necessary to change the output rates of pumps of this type, it can be done, in one case, by providing equipment for changing the speed at which the respective pump is being driven or, in another case, in pumps of the type here under consideration where the pumping action is the result of reciprocation of at least one piston or plunger within and with respect to an associated cylinder, by providing equipment capable of adjusting the length of the stroke of the piston or plunger. The conventional equipment of this type is very complex and hence expensive.
Another possibility of adjusting the output rate of the pump of the type discussed here is to resort to the use of bypass valves in the discharge or high-pressure line of the pump or within the pump itself. As a matter of fact, because of the relative simplicity and inexpensiveness of the equipment used for achieving this expedient, this solution to the problem of adjusting the output rate of the plunger-type high-pressure pumps is currently in widespread use. Under these circumstances, an amount of the fluid being pumped which is not needed for the operation of the device to which the fluid is being supplied flows through this bypass valve back to the suction side of the pump, or more particularly into an appropriate reservoir from which the pump withdraws the fluid, and the fluid is being heated during its passage through the bypass valve and the conduits leading to the suction side. Of course, the conversion of the mechanical energy into thermal energy is disadvantageous for several reasons. First of all, the increased heat content of the fluid is usually undesirable since the consistency or other properties of the fluid may be adversely affected thereby. Thus, it is often necessary to cool the fluid before it is readmitted to the pump which, in turn, results in thermal pollution of the environment. However, even in the absence of cooling by a special cooling equipment, the increased heat content of the fluid will be dissipated into the environment, with similar consequences. Last but not least, the amount of energy which is converted into thermal energy is to be considered to be wasted in most instances, so that the overall efficiency of the pump is reduced.
When the pumps of the above type are being used for rapid approach of presses, during the pressing of protective caps against the mine roof in mining operations, during the advancement of the lining and of the mining machinery also in mining operations, it is necessary to bridge idle intervals during which the lifting cylinder-and-piston unit is to be filled. Under these circumstances, only the frictional resistance of the various parts of the equipment is to be overcome at first. In order to obtain short time intervals for these phases of the operation of the equipment, it is necessary or advantageous to supply the equipment with large amounts of the working medium during these phases of operation. Then, after the rapid-approach phase, there follows a period of time during which the pressure of the working medium which is required for operating the equipment is built up to the desired level which is then being maintained over an extended period of time.